GTX 275 stomps the 4870, the GTX 275 is very close to the GTX 285 in terms of performance but is $100 less, you could also take a look at the 4890 but from what ive seen the GTX 275 is a bit faster and similarly priced.

Honestly I would spend the extra and get a WD 640gb black hard drive, its not so much about the extra space as it is the speed of the drive, they are very fast and very affordable. also the TX750 will easily be able to handle a dual card setup, it MIGHT struggle with 2 GTX 275s, but I know it can run 2x GTX 280s so I think 2 GTX 275s would be easily handled.

Oh and theres a review of the GTX 275 as well as the 4890 on this site..